Scientists have, for the first time, demonstrated a direct link between the amount of mercury in rain and the amount of toxic methylmercury that accumulates in fish in an experimental lake in Canada.

Most mercury in the atmosphere today is generated from human activity such as power plant emissions, and industrial and mining activities. Understanding the impacts of that type of mercury deposition on mercury levels in fish has been poorly understood, however. Natural bacteria found in the sediment of lakes and wetlands convert it to an organic form known as methylmercury, which accumulates in fish tissue and is toxic to humans and fish-eating wildlife. High levels of methylemercury have been found in fish throughout North America and have prompted health authorities to issue advisories against fish consumption in most areas. A study published this week in the journal Environmental Science and Technology suggests that reducing the amount of mercury deposition should proportionally reduce the amount of methylmercury production, thereby reducing the methylmercury available to accumulate in fish.

It is an important finding because many areas already have large historical accumulations of mercury in sediments and soils, and it has been difficult to distinguish between the impacts of historical contamination and new deposition. “This study clearly links current deposition of mercury with methylmercury production in lakes” said Dr. Cynthia Gilmour, Microbial Ecologist at the Smithsonian Environmental Research Center (SERC) and one of the authors on the study.

Gilmour and her colleagues examined the response to changing mercury loads using a set of lake enclosures called mesocosms. The mesocosms were dosed with a range of mercury concentrations that represent the range of atmospheric mercury deposition rates in North America. The study was located in the Experimental Lakes area in northwestern Ontario, Canada. Individual mercury stable isotopes were used to separately track the experimental mercury loads. As they tracked the fate of their mercury through the system, they found that though much of it was re-emitted to the atmosphere, a small percentage was converted to methylmercury. “It may be a tiny amount, but that is all it takes to drive those fish advisories,” said the study's lead author Diane Orihel of the University of Manitoba and Fisheries and Oceans Canada.

This study suggests that reducing mercury emissions from power plants, as the U.S. EPA has required in its March 2005 cap and trade rule (the Clean Air Mercury Rule), should result in proportional reductions in methylmercury production and bioaccumulation, particularly in ecosystems that receive most of their mercury loading to the surface of water bodies. For more information about Dr. Gilmour's work or to reach Dr. Gilmour, please contact SERC science writer cutlipk@si.edu